Chemical processes related to net ozone tendencies in the free troposphere

Abstract. Ozone (O₃) is an important atmospheric oxidant, a greenhouse gas, and a hazard to human health and agriculture. Here we describe airborne in situ measurements and model simulations of O₃ and its precursors during tropical and extratropical field campaigns over South America and Europe, respectively. Using the measurements, net ozone formation/destruction tendencies are calculated and compared to 3-D chemistry–transport model simulations. In general, observation-based net ozone tendencies are positive in the continental boundary layer and the upper troposphere at altitudes above  ∼  6 km in both environments. On the other hand, in the marine boundary layer and the middle troposphere, from the top of the boundary layer to about 6–8 km altitude, net O₃ destruction prevails. The ozone tendencies are controlled by ambient concentrations of nitrogen oxides (NO_x). In regions with net ozone destruction the available NO_x is below the threshold value at which production and destruction of O₃ balance. While threshold NO values increase with altitude, in the upper troposphere NO_x concentrations are generally higher due to the integral effect of convective precursor transport from the boundary layer, downward transport from the stratosphere and NO_x produced by lightning. Two case studies indicate that in fresh convective outflow of electrified thunderstorms net ozone production is enhanced by a factor 5–6 compared to the undisturbed upper tropospheric background. The chemistry–transport model MATCH-MPIC generally reproduces the pattern of observation-based net ozone tendencies but mostly underestimates the magnitude of the net tendency (for both net ozone production and destruction).